Metal Oxide Semiconductor Field Effect transistors (MOSFETs) are common transistors used in digital and analog circuits. MOSFETs include a channel of n-type or p-type semiconductor material through which current flows. MOSFETs that are fabricated to include channels of n-type semiconductor material are called NMOSFETs and MOSFETs that are fabricated to include channels of p-type semiconductor material are called PMOSFETs. MOSFET types include planar MOSFETs and trench MOSFETs. Trench MOSFETs have facilitated an increase in attainable circuit density because of their small cell pitch as compared to planar MOSFETs.
The current that flows through the MOSFET, e.g., through the channel from drain to source (or vice versa), may be influenced by factors that include the physical design of the MOSFET. More specifically, aspects of the physical design such as the length of the MOSFET channel help to determine current related parameters such as the conductance and the resistance of the MOSFET channel (the resistance of the MOSFET is controlled by the channel after the MOSFET turns on). Importantly, these parameters are critical to the performance of the MOSFET as measured by the speed of operation and the power dissipation of the device. It should be appreciated that low channel resistance (such as is provided by shorter channel lengths) reduces power dissipation and increases device efficiency.
Conventional approaches to improving MOSFET performance include efforts to shorten the length of the MOSFET channel. The channel dimensions of a MOSFET may be defined by its junctions (e.g., interfaces between n-type and p-type regions) which may be established by the diffusion of impurities into the body of the device. Characteristics of the diffusion such as the depth of the diffusion help to establish the length of the channel. In fact, in some conventional processes the depth of diffusion alone may determine the length of the channel. In other conventional processes, the depth of the MOSFET trench may be a factor in determining the length of the channel.
It should be appreciated that in some cases the depth of diffusion and thus the length of the MOSFET channel may be controlled through appropriate management of implant energy and temperature. However, at small distances problems may arise as material variation may have a heightened impact on diffusion characteristics. And, as diffusion characteristics become more difficult to manage, the establishment of channel length becomes more difficult to control through diffusion.
In other conventional approaches attempts have been made to reduce channel length beyond that which is attainable through diffusion alone. In one such approach MOSFET trench depth has been used to achieve very short channel lengths. However, in such cases the relationship of the depth of the MOSFET trench to the depth of the p type regions formed in the body of the MOSFET structure (e.g., such as the contact implant, the contact clamping implant and body implant) needs to be carefully managed. Importantly, where the trench bottom is shallower or comparable in depth to the p-type regions formed in the MOSFET body then significant pinching may occur.
FIG. 1A shows a conventional trench MOSFET 100. When the gate 103 to source 101 voltage (e.g., Vgs) of MOSFET 100 is greater than a threshold voltage, then current flows in MOSFET 100 from drain 105 to source 101. In order to scale the devices to shorter channel-lengths and shallower trench depths, the trench-bottom implant, in which an n-type dopant (for n-channel devices) is implanted at the trench bottom so as to compensate for the p-type body implant and defining the channel bottom, had been found to provide very short channel lengths. Since this implant is self-aligned to the trench bottom, the channel-length is determined by the trench depth.
It should be appreciated that p-type body implant 107 may move so as to cover the bottom of trench 103 as is shown in FIG. 1B (see dashed line). Such covering of the bottom of trench 103 or “pinching” may substantially increase the resistance (e.g., rdson) of the channel of MOSFET 100 and may significantly degrade the MOSFETs performance. In other words, significant pinching occurs at trench depths such that the trench bottom is shallower as comparable in depth to the p-type region defined by the contact and contact-clamping implants, resulting in high rdson. Conventional techniques that facilitate the attainment of short channel lengths that are defined through the use of trenches do not include measures that effectively address such pinching that may occur at very short trench depths.